HF-VHF-UHF IQ Mixer with a Single SPDT Switch

Rick Campbell

Portland State University, Portland OR 97201

This Talk:

Technical details of the mixer

Our Applications

Experimental Platform

Simulation Platform

What we’ve learned so far

....Next Steps

Application: Off-Grid Wireless...seriously

Wind Power, Environmentally Conscious

Starved Current

Student experiments run on 9v battery

Short path from sketch through prototype circuitry, then deploy measurement instruments

Low Cost

Attracts an interesting community of students from diversebackgrounds and all genders.

“What voltage zener makes a better tuning diode?”

Overheard:

Frequency Conversion 101

High Frequency Low Frequency

Multiply by 1 or 0

If Low Frequency V is plus or minus dc, then RF comes out the�  High�  Frequency�  port,�  at�  potentially�  very�  high�  efficiency�  depending on reactive loading. Stored Energy...

Our basic mixer unit cell V

HF inHF out

Redraw:

Use a pair of unit cells in an IQ mixer with SPDT switch:

HighFrequency RF

LowFrequency I

Note: bidirectional, no attempt at optimum drive or load at switch.�  �  Could�  benefit�  from�  some�  waveform�  engineering...

basic conceptual I Q cell

LowFrequency Q

First Pass Waveform Engineered version

Design network components using classic Smith Chart techniques to optimize switch impedance environment

RF I

Q

Examples: Higher Z to reduce effect of On resistance, lower Z to minimize switch varactor effects, etc.

Next, Simulations and Lab Experiments

Simulation�  first:�  ideal�  switch�  exists�  in�  the�  simulator.

RF I

Q

a

b

Note: component values in IMS paper

a

b

Simulation Schematic

all passive baseband signal processor

I

Q

Experimental Platform:

Switch candidates go on the back, alternately connecting points a and b to ground

RF

Note:�  7�  MHz�  low�  frequency�  test�  fixture�  allows�  connecting oscilloscope to observe waveforms

I

Q

a

b

LO

a

b

a

bLO

Two different SPDT switches built on the back of the experimental I Q mixer board

VN10 MOSFETs with gate bias

Agilent 5082-2835 diode ring andhybrid coil

bias

a

b

a bLO LO

bias

Two different SPDT switch schematics

Two MOSFET SPDT switch

Hybrid coil and diode ring SPDT switch

Summary of Results: Note I Q stability and 2nd Order

MOSFET mixer typically has several mV of dc offset at I and Q ports.Variable with LO drive and RF signal level. Various 2nd order tests.

�  �  3rd�  order�  products�  with�  ideal�  switch�  are�  above�  the�  simulation�  noise�  floor

Measured opposite sideband suppression typically degraded by 10 dB for a few dB change in LO drive level--several degree phase shift.*

*

SSB Loss dB

IIP3 dBm

LO dBm

2nd Order

I Q stability vs LO drive

Ideal Switch VN10 MOSFET Hybrid/Diode

Simulated vs Measured Performance

4.1 5.3 5.9

+22.8 +21.2 +6.9

N/A +10 +4

in simulation noise floor

depends on FET model

>40 dB LSB suppression

6.2

+16.5

+17

several mV of dc offset

poor

6.4

+6.2

+4

no change for LO0dBm to 10dBm

sim simmeasured measured

*

>40 dB LSB suppression

>40 dB LSB suppression

in simulation noise floor

in measurement noise floor

*

sim

Measured MOSFET 2nd order performance is highly variable and always poor--may be related to gate capacitor varactor

I Q stability vs LO drive also likely due to gate cap varactor

Simplified�  MOSFET�  model inserted into simulator�  with�  fixed�  gate capacitor and on resistance adjustedto match measuredconversion loss

With�  simplified�  model, 2nd order performance and IQ stability are ideal

Note that Off pair of diodes in the ring is clamped to precisely the forward voltage of the On diodes:

diode�  reverse�  capacitance�  clamped�  to�  fixed�  value-­-­good�  for�  2nd order balance and IQ stability.

Two�  significant�  implications:

Off diodes have relatively small reverse bias--limits 3rd order IM performace: 350 mV reverse voltage, 100 mV signal...

Conversion Loss and Sideband Suppression Measurements

LO level

passive basebandsignalprocessor

Measurements are basic and calibrated by substitution

Higher�  Frequency�  tests.�  �  Start�  with�  a�  modified�  ADE-­1�  diode ring mixer:

Interesting manual work under the microscope. Lead pitch is 2.5�  mm.�  �  This�  required�  tweezers�  and�  a�  fine�  soldering�  iron�  tip.

144 MHz same schematic, scaled reactances, physically smaller parts

SMC�  trifilar�  transformerair-core hybrid transformer

surface mount parts and diode pairs

VHF surface mount versions have no surprises. A family of examples compared with conventional I Q diode ring pairs.

VHF #2

HF reference

VHF #1

Compared with reference: more modest IM; very good I Q stability; comparable loss; lower 0 dBm LO drive.

Summary

A simple and different I Q mixer topology has been explored using a platform that facilitates experiments with different switches.

Competitive conversion loss of 6 dB and modest IM performance at 0 dBm LO drive are attractive for simple battery power applications at HF and VHF.

The only active component is a single SPDT switch, which facilitates frequency scaling.

The mixer specs for frequency conversion are not the most important specs for I Q applications.

Next Steps:

Clearly already too much work to squeeze into a conference�  paper.�  �  If�  there�  is�  sufficient�  interest�  a�  more complete Transactions paper will be prepared.

Unlike�  broadband�  mixer�  topologies,�  band-­specific�  mixers present opportunities for optimization using waveform engineering methods.

We have already begun work on very low power I Q receivers using 0 dBm LO power and the modest IM performance offered by this topology.

THz and milliHertz versions are possible with an appropriate SPDT switch.

Thank you!

Note: bow is low. Minutes after photo, discovered that prior generation electronics was soaked in salt water.

hence these experiments...

2013 photo